Optimization and Kinetics of Bioadsorption of Aqueous Potassium Permanganate in Charred Cacao Shells

In this study, charred cacao shells were tested for their capability as bioadsorbent for aqueous potassium permanganate. Optimization of adsorption parameters showed that the best amount of adsorbent is 0.5% w of cacao biochar/v of 0.005 N aqueous KMnO4 with a removal efficiency of 98.79%, and the optimum particle size is 212 m, with a corresponding removal efficiency of 99.76%. Kinetics studies showed that the adsorption fits a pseudo-first order reaction with R = 0.9833. Lastly, the adsorption data fits the Elovich isotherm with R = 0.9712, which strongly suggests a multi-layer chemisorption on a heterogenous surface. Conclusively, the charred cacao shells were found to be effective as an adsorbent of aqueous potassium permanganate. Furthermore, the resulting product of adsorption can be used to study adsorption of other substances due to its multi-layer nature.


INTRODUCTION
Cacao (Theobroma cacao L.) is one of the high value crops in the Philippines, with its annual consumption of around 50,000 metric tons. On top of that, it is an important agricultural commodity marketed worldwide, whose demand is projected to reach 500,000 metric tons by 2020 (Department of Agriculture-Bureau of Plant Industry, 2017). Along with the rising demand for cacao beans and their products are the increasing amount of wastes generated through the different processes they undergo. Upon reaching maturity cacao pods are harvested, followed by the removal of the beans and adhering pulp for further processing, such as fermentation and roasting. The cacao bean contains polyphenols such as catechins, proanthocyanidins, and anthocyanins, which gives the bean a red to purple color. Upon roasting, these polyphenols are oxidized, forming high molecular weight tannins (Romero-Cortes et al., 2013). Fermented cacao beans are then Volume 31, Number 2, July 2020 • KIMIKA subsequently used for the manufacture of chocolate and other cacao-based products, while the inedible cocoa pod husk, bean shell, and cocoa mucilage were underutilized and even treated as wastes (Panak Balentic et al., 2018). The residual biomass, still rich in protein, fiber, ash, and phenolics, oftentimes produces foul odors if not properly stored and/or disposed.
Recently, cacao shells were found to have applications in fertilizer and animal feed preparations, but are mainly used as fuel for boilers (Fowler and Coutel, 2017). Moreover, phenolics-rich extracts obtained from cacao shells were found to have applications in biogas production (Mancini et al., 2016). Cacao shells were also utilized as starting material in an adsorption study for the treatment of lead, cadmium, and other metals (Osakwe et al., 2014;Tejada-Tovar et al., 2018) This study focused on the potential use of processed cacao shells as adsorbent in treating oxidizing wastes, more specifically, potassium permanganate. Mainly used as an antiseptic, potassium permanganate is a highly corrosive and water-soluble strong oxidizing agent, with a lethal adult dose of 10 g/kg (Agrawal et al., 2014). One main concern for the disposal of potassium permanganate wastes is its toxicity to aquatic organisms such as fishes, as it is usually used as disinfectant in fish hatcheries. Exposure to 25, 50, 100, 150, and 200 mg/L potassium permanganate of tilapia eggs could lead to 0 % hatching rate (Hanjavanit et al., 2013). Further studies lead to calculation of the 96h-LC50 for Caspian kutum (Rutilus frisii kutum), which is 3.147-3.260 mg/L and 3.393-3.528 mg/L for small and large fishes, respectively (Nodeh and Hoseini, 2013).
In the academe, potassium permanganate solutions are often used in laboratory experiments in varying concentrations. Over time, a laboratory can accumulate a high volume of wastes, which poses a problem since permanganate is a strong oxidizing agent. The use of modified carbonaceous material was found to effectively remove potassium permanganate from drinking water, seawater, and industrial wastewater (Mahmoud et al., 2016). The development of other materials and procedure is beneficial in order to have a protocol to treat such wastes to prevent its toxicity and minimize its amount in the environment.
Several researches have delved deeper into the use of permanganate as a surface modifier to improve adsorption (Hu et al., 2018, Pang et al., 2015, Mopoung and Bunterm, 2016. A KMnO4modified activated carbon was effective in adsorbing formaldehyde with a pseudo-second order rate reaction (Hu et al., 2018). At low pH, activated carbon modified with increasing KMnO4 concentrations were found to have higher percentage removal of aqueous Cr (VI) compared to the unmodified adsorbent (Pang et al., 2015). Moreover, modification of the carbon obtained from pineapple leaf fibers by potassium permanganate, increased its ferric ion removal efficiency (Mopoung and Bunterm, 2016). These findings strengthen the possibility of developing and utilizing modified adsorbents to become more effective at removing heavy metals and other wastes.
The aim of this study was to optimize the parameters for the adsorption of potassium permanganate in charred cacao shells and to determine the governing mechanism using different adsorption isotherms.

METHODOLOGY
Materials and Equipment. Charred cacao shells were obtained from Filipinas Oro de Cacao, Inc. on 23 October 2019 as a by-product of their manufacturing process. Potassium permanganate was purchased from JT Baker. The aqueous KMnO4 solution was boiled, filtered and stored in an amber bottle before use. For the equipment, a Shimadzu IR Spirit with a diamond ATR accessory was used for FTIR analysis and the pH of the solutions were determined using a pH meter. A KIMIKA • Volume 31, Number 2, July 2020 Shimadzu AA-7000 GFA with a lamp wavelength of 279.5 nm, 7.0 mm burner height at 2.0 L/min acetylene and 15.0 L/min air, was used for Mn content analysis. A W.S. Tyler R-30050 sieve shaker was used for particle size separation. The analyses were performed in the Analytical Services Laboratory, Institute of Chemistry and E3 laboratory of the College of Engineering and Agro-Industrial Technology, University of the Philippines Los Baños.
Sample preparation. The charred cacao shells were air-dried for 24 hours followed by oven drying at 90°C until moisture content is < 1%. The dried biomass was then sieved through an 850 µm, 425 µm, and 212 µm mesh using a sieve shaker. The separated fractions were weighed to get the percentage of each particle size range.
Characterization of charred cacao adsorbent. The pH of the charred cacao adsorbent was determined by mixing 0.250 g with 50.0 mL distilled water and shaken for two hours. The pH of the resulting mixture was determined at 28 o C. The FTIR spectra of the charred cacao adsorbent was determined by placing the dried cacao biochar sample directly on the ATR and analyzed for existing functional groups.

Effect of amount of adsorbent.
To determine the effect of the amount of adsorbent on the adsorption of KMnO4, 0.125, 0.250, and 0.500 g of the 212-µm charred cacao adsorbent were placed in separate Erlenmeyer flasks containing 50.0 mL 0.005 N KMnO4 solution. The flasks were placed in an orbital shaker at 150 rpm for 120 mins under ambient temperature. The mixtures were filtered through a Whattman no. 5 filter paper and the resulting filtrates were analyzed for Mn content using a Flame Atomic Absorption Spectrophotometer (FAAS). Triplicate measurements were done for each sample.

Effect of particle size.
To determine the effect of particle size on the adsorption of KMnO4, 0.250 g of 212, 425, and 850 µm of charred cacao adsorbent were placed in separate Erlenmeyer flasks containing 50.0 mL 0.005 N KMnO4 solution. The flasks were placed in an orbital shaker at 150 rpm for 120 mins under ambient temperature. The mixtures were filtered through a Whattman no. 5 filter paper and the resulting filtrates were analyzed for Mn content using FAAS. Triplicate measurements were done for each sample.
Determination of percent removal. The percentages were calculated using equation 1, where Co is the initial concentration of KMnO4 used and Ce is the concentration of KMnO4 at equilibrium.
Determination of kinetics of adsorption and the adsorption isotherm. The kinetics study was done using the optimized and upscaled parameters as described by (Tejada-Tovar et al., 2018). Adsorption rate was monitored by placing 1.50 g of cacao biochar in a 500-mL Erlenmeyer flask containing 300.0 mL 0.005 N KMnO4 solution. The flasks were placed in an orbital shaker at 150 rpm for 120 mins under ambient temperature. For the first 30 minutes, 3-mL aliquots were taken at 10-minute intervals. The aliquots were centrifuged, and the supernatant was acidified and diluted 1:10. After 30 minutes, the time interval was changed to 15 minutes until 120 minutes. The diluted aliquots were analyzed for Mn using FAAS.
Both the pseudo first-order and pseudo second-order analyses were used to determine the rate of adsorption, while the adsorption isotherm was determined using the Langmuir, Freundlich, Dubinin-Radushkevich (D-R), Flory-Huggins, Temkin, and Elovich models.  Figure  1) showed that the spectrum is similar to that of non-starchy polysaccharides such as lignin due to the O-H stretch and aromatic C=C stretch (Derkacheva and Sukhov, 2008). This suggests that the cacao biochar has not been fully charred and still contains organic plant materials that may have played a role in the adsorption of KMnO4. Characterization of the cacao biochar showed that it produces an alkali solution of pH 10 when mixed with water. Based on this data, if KMnO4 undergoes chemisorption in the mixture, the resulting product is brown MnO2 solid (see Equation  2). This was consistent with the color change observed in the adsorbent, which changed from black to brown after the allowed time interval. As observed with other studies, a smaller particle size will increase the efficiency of adsorption due to an increase in exposed surface area for the compound of interest to adhere to (Foiresi et al., 2018). On the other hand, the effect of varying mass of adsorbent with a constant concentration of sorbent showed that 0.250 g of cacao biochar was the optimum amount in removing KMnO4 in a 50.0 mL of 0.005 N solution. Using 0.250 g of cacao biochar removed 98.79 % KMnO4 in the solution while using 0.125 g and 0.500 g of cacao biochar removed only 83.80 % and 94.08 %, respectively. Although using 0.500 g of cacao biochar is still considered efficient, it was observed that increasing the amount of cacao biochar resulted in a more colored mixture. This led to the conclusion that 0.05 g of cacao biochar per mL of 0.005 N KMnO4 was the optimum proportion. An increase in mass may be correlated to the increase in the number of particles and consequently, the increase in the number of sites for adsorption. However, it was observed that further increasing the amount did not lead to significant changes in the removal efficiency. The use of cacao shell for Pb and Cd adsorption was also consistent with this observation (Osakwe et al., 2014).

Kinetics of adsorption.
The amount of KMnO4 remaining in the mixture at certain time intervals for 60 mins was determined to study the effect of contact time on the adsorption. As expected, the removal efficiency increases with time and was found to be complete after 60 mins since there wavenumber, cm -1 KIMIKA • Volume 31, Number 2, July 2020 was no Mn detected in the mixture (Figure 2). From this, the adsorption kinetics of KMnO4 using cacao biochar was determined by regressing the data unto linearized pseudo first-order (Equation 3) and pseudo second-order (Equation 4) kinetic equations (Marquez and Ramos, 2019).   Based on the kinetic models as seen in Table 1, the adsorption process follows a pseudo first-order rate equation (Figure 3) with a correlation coefficient (R 2 ) of 0.9833 as compared to the pseudo second-order rate equation (Figure 4) with a R 2 of 0.5826. A pseudo first-order reaction is a second order reaction whose rate depends on the concentration of two of the reactants. In the experiment, the concentration of KMnO4 was kept constant, so the rate of adsorption process depends on the adsorbent. This is different from the results of Aprilliani and colleagues (2018) where pseudo second-order rate reaction was observed for the adsorption of KMnO4 using coconut shell-derived activated carbon. It can be inferred that biochar follows a different rate compared with activated carbon.

Selection of Appropriate Adsorption Isotherm. Determination of adsorption rates and their
corresponding mechanism are important especially in designing adsorption experiments, which could be applied to large-scale treatments of industrial wastes (Foo and Hameed, 2010). Adsorption isotherms models can help predict the mechanism of the adsorption process in terms KIMIKA • Volume 31, Number 2, July 2020 of stability and thermodynamic favorability (Marquez and Ramos, 2019). Given the kinetic parameters of the experiments, the adsorption isotherm model amongst Langmuir, Freundlich, Dubinin-Radushkevich, Flory-Higgins, Temkin, and Elovich were determined. The equations for the different adsorption models are summarized in Table 2, and the corresponding correlation coefficient and mechanism inference are summarized in Table 3. The results showed that Elovich was the isotherm that best fits the adsorption data with a R 2 of 0.9712. Table 2. Summary of isotherm linearized equations and corresponding terms used in this study. Adapted from Marquez and Ramos, 2020.

Isotherm Equation Terms
Langmuir (  The Elovich model assumes that the adsorption sites increase exponentially with adsorption (Ayawei, Ebelegi and Wankasi, 2017). According to this model, mechanism of adsorption is based on chemical reactions which are responsible for adsorption. Therefore, there is an exchange of species on the non-homogeneous surface of the solid, which can be summarized as a multi-layer type of adsorption. This result is consistent with previous kinetic studies of Tejada-Tovar et al (2018) in which cacao shells have heterogenous surfaces, common to agricultural biomasses with complex composition. In addition to that, it supports the theory that the mechanism is chemisorption, where the permanganate (Mn VII) is reduced to manganese dioxide (Mn IV) upon adsorption. Moreover, the isotherm suggests a multi-layer adsorption which could mean that the adsorption of MnO2 makes the material become a better adsorbent.
In the study of Eduah and colleagues (2020), the adsorption of phosphate using cocoa pod husk, corn cob, rice husk, and palm kernel shell biochar was explained using the Elovich model. Another researcher used the Elovich model to explain the adsorption of atrazine and imidacloprid using eucalyptus bark, corn cob, bamboo chips, rice husk, and rice straw biochar (Mandal et al., 2017). The result of this study is consistent with the results of other researchers suggesting that the Elovich isotherm best explains the adsorption models of most agricultural by-product derived biochars.
In the study of Ezuego and Anadebe (2018) where they used animal bone and corn cob-derived activated carbon in removing KMnO4, Freundlich and Langmuir isotherms were observed, which indicates a favorable adsorption for electrostatic interaction between the adsorbent forces and KMnO4. Although physiosorption was the main mechanism of adsorption, the researchers also acknowledged the possibility of slight chemisorption.
Comparing the results of Ezuego and Anadebe (2018) and this study, it can be observed that mechanism of adsorption of the activated carbon is physiosorption while biochar resulted in a combination of both chemisorption and physiosorption. This is because the biochar has not been fully converted into carbon and still contain organic plant materials such as cellulose and lignin. These organic materials may be responsible for the chemical reactions involved during the adsorption process.

CONCLUSION
Based on the results, the optimum condition of adsorption involved the 212 µm fraction and a proportion of 0.05 g of cacao biochar per mL of 0.005 N KMnO4 solution with a maximum removal of 99.76 % at 60 mins. The adsorption mechanism was found to be pseudo first-order reaction, which states that the reaction rate depends on the concentration of two of the reactants, since it has a higher correlation coefficient (R 2 ) of 0.9833 as compared to the pseudo second-order rate equation with a R 2 of 0.5826. The appropriate adsorption isotherm was also determined by fitting the models Langmuir, Dubinin-Radushkevich, Flory-Higgins, Temkin, and Elovich with the data. It was found that Elovich was the best fit amongst the isotherms, which had a R 2 of 0.9712. The mechanism of adsorption is a combination of chemisorption and physiosorption, where the permanganate (VII) is reduced to manganese dioxide (IV) which then adsorbs to the cacao biochar. Further study of this is suggested for its application in mixed oxidant wastes and use of the resulting residue as an adsorbent for other wastes.